Patterns and Mechanisms of Evolution Natural selection is not the only mechanism of evolution. EQ: What patterns can be observed in evolution and what factors influence speciation? Copyright © McGraw-Hill Education Shaping Evolutionary Theory

Patterns of evolution These are models of evolution: A. Mass Extinctions B. Adaptive Radiation C. Convergent Evolution D. Coevolution E. Gradualism F. Punctuated Equilibrium

A. Mass Extinctions Event in which many types of living things became extinct at the same time.  Huge numbers of species disappeared. Whole ecosystems were wiped out. Resulted in burst of evolution of new species in new habitat Disrupted energy flow throughout the biosphere and caused food webs to collapse (Possible Causes: Asteroids, volcanic eruptions, continental drifts, changing sea levels)

B. Adaptive Radiation (Divergent Evolution) The evolution of an ancestral species, which was adapted to a particular way of life, into many diverse species, each adapted to a different habitat Many new species diversify from a common ancestor . The branching out of a population through variation. The new species live in different ways than the original species did.

C. Convergent Evolution Opposite of divergent evolution (adaptive radiation) Unrelated organisms independently evolve similarities when adapting to similar environments, or ecological niches Analogous structures are a result of this process Example: penguin limb/whale flipper/fish fin The wings of insects, birds, pterosaurs, and bats all serve the same function and are similar in structure, but each evolved independently

D. Coevolution The mutual evolutionary influence between two species Bumblebees and the flowers the they pollinate have co-evolved so that both have become dependent on each other for survival. The mutual evolutionary influence between two species When two species evolve in response to changes in each other They are closely connected to one another by ecological interactions (have a symbiotic relationship) including: Predator/prey Parasite/host Plant/pollinator Each party exerts selective pressures on the other, thereby affecting each others' evolution

E. Gradualism The evolution of new species by gradual accumulation of small genetic changes over long periods of time Emphasizing slow and steady change in an organism Occurs at a slow but constant rate Over a short period of time it is hard to notice Current living zebras (top), extinct quaggas (bottom)

F. Punctuated Equilibrium Stable periods of no change (genetic equilibrium) interrupted by rapid changes involving many different lines of descent Opposite of gradualism It is rare, rapid events of branching speciation Characterized by long periods of virtual standstill ("equilibrium"), "punctuated" by episodes of very fast development of new forms Horseshoe crabs have changed little since their first appearance in the fossil record. They are in a state of equilibrium

Gradualism vs Punctuated Equilibrium

Mechanisms of Evolution-Evolution occurs at the population level Population genetics Hardy and Weinberg showed that evolution will not occur in a population unless allele frequencies are acted upon by forces that cause change. Hardy-Weinberg principle states that when allele frequencies remain constant, a population is in genetic equilibrium. Copyright © McGraw-Hill Education Shaping Evolutionary Theory

Mechanisms of Evolution Population genetics For a population to be in genetic equilibrium according to the Hardy-Weinberg principle, it must meet five conditions: No genetic drift No gene flow No mutation Mating must be random No natural selection These five conditions are the mechanisms of evolutionary change and VIOLATE the Hardy-Weinberg Principle Copyright © McGraw-Hill Education Shaping Evolutionary Theory

Mechanisms of Evolution 1. Genetic drift Any change in the allele frequencies in a population that results from chance is called genetic drift. In smaller populations, the effects of genetic drift become more pronounced, and the chance of losing an allele becomes greater. Copyright © McGraw-Hill Education Shaping Evolutionary Theory

Genetic drift The founder effect results when a group of individuals with a different allele frequency than the original population becomes isolated. Alleles that were infrequent in the original population may be common in the new population. Happens when a subset of organisms settles in an area separated from their original population result in large genetic variations in the separated population Copyright © McGraw-Hill Education Shaping Evolutionary Theory

Genetic drift A bottleneck results when population declines (due to a catastrophe) to a very low level and then rebounds. The gene pool of the rebound population is similar to the low- level population, which may have reduced diversity. Bottleneck Effect: Northern Elephant Seal Population Hunted to near extinction Population decreased to 20 individuals in 1800’s, those 20 repopulated so today’s population is ~30,000 No genetic variation in 24 genes Copyright © McGraw-Hill Education Shaping Evolutionary Theory

2. Gene flow 3. Nonrandom mating 4. Mutation Mechanisms of Evolution Uncommon in natural populations – organisms migrate/move between populations Random movement increases genetic variation within a population and decreases the genetic variation between populations. 3. Nonrandom mating A population in genetic equilibrium must exhibit random mating. This rarely occurs in populations – mating occurs between individuals in close proximity Promotes inbreeding Changes allele frequencies, favoring individuals that are homozygous 4. Mutation The cumulative effect of mutations in a population might shift allelic frequencies. Most mutations are harmful, but some may be beneficial and become more common. Copyright © McGraw-Hill Education Shaping Evolutionary Theory

5. Natural selection Mechanisms of Evolution Natural selection acts to select the best adapted for survival and reproduction. Three types of natural selection: Stabilizing-eliminate extreme expressions of a trait Directional-increases the expression of an extreme version of a trait Disruptive-removes individuals with average traits Sexual selection -change in the frequency of a trait based on the ability to attract a mate. Copyright © McGraw-Hill Education Shaping Evolutionary Theory